1. Field of the Invention
The invention relates generally to rotary drag bits, and more specifically to improvements in roof drill bits for drilling and boring as in roof bolting operations for tunnel construction and mining.
2. Description of the Prior Art
In the fields of industrial, mining and construction tools, polycrystalline diamond (PCD) is becoming more widely used in making cutting tool inserts, sometimes called polycrystalline diamond compacts (PDC). PCD materials are formed of fine diamond powder sintered by intercrystalline bonding under high temperature/high pressure diamond synthesis technology into a predetermined layer or shape; and such PCD layers are usually permanently bonded to a substrate of "precemented" tungsten carbide to form such PDC insert or compact. The term "high density ceramic" (HDC) is sometimes used to refer to a mining tool having an insert with a PCD layer. The term "chemical vapor deposition" (CVD) is a form of pure PCD used for inserts that are denser and last longer in use in the mining field. Other hard surfacing and layered materials, such as layered "nitride" compositions of titanium (TiN) and carbon (C.sub.2 N.sub.2), are gaining acceptance in the mining field. All such "hard surface" materials--PCD, CVD and nitride compositions are applicable to the present invention and considered as alternatives unless specifically distinguished from each other herein.
Some of the basic underlying technology pertaining to PCD materials is disclosed in U.S. Pat. Nos. 4,525,178; 4,570,726; 4,604,106; and 4,694,918. In particular, U.S. Pat. No. 4,570,726 discloses special insert shapes for coring-type rotary drill bits, and suggests a tool having a curved working surface positioned at a slight negative rake angle from the axis of rotation (see also U.S. Pat. No. 4,858,707). The use of PCD materials in rotary earth drilling equipment replaces the long time use of tungsten carbide or the like as an abrasive cutting material; and most developmental work in PCD/CVD rotary drilling has been in the oil/gas field involving deep well boring into the earth's crust.
The principal types of drill bits used in rotary drilling operations are roller bits and drag bits. In roller bits, rolled cones are secured in sequences on the bit to form cutting teeth to crush and break up rock and earth material by compressive force as the bit is rotated at the bottom of the bore hole. In drag bits, PCD cutting elements on the bit act to cut or shear the earth material. The action of some flushing medium, such as fluid drilling mud or compressed air, is important in all types of drilling operations to cool the cutting elements and to flush or transport cuttings to the upper surface of the well. It is important to remove cuttings to prevent accumulations that will "ball up" or otherwise interfere with the crushing or cutting action of the bit and the cooling action is particularly important in the use of PCD/CVD cutters to prevent carbon transformation of the diamond material. In deep well drilling the circulation of drilling mud is contained in the well bore hole and can be recaptured and controlled at the well surface. U.S. Pat. No. 5,358,063 discloses a deep well drill bit having a series of hard material button inserts, and the invention pertains to improvements in transporting the flushing medium (compressed air) to prevent erosion around and loosening of the inserts.
Although roof drill bits are a form of rotary drag bit, it will be recognized that there are vast differences from deep well drilling. Roof bolting operations are overhead so the drilling operation is upward rather than downward, and in most cases the earth structure is formed of extremely hard rock or mineral (coal) deposits, although stratas of shale, loose rock and mud layers are frequently encountered in boring (drilling) operations for roof bolting construction. The use of large quantities of water (drilling mud) is typical in roof drilling to cool the cutting elements and flush the cuttings away, but overhead irrigation results in uncontrolled water loss and floor flooding that make working conditions unsafe and unpleasant. My prior U.S. Pat. Nos. 5,180,022; 5,303,787 and 5,383,526 disclose substantial improvements in HCD roof drill bits using PCD cutting elements constructed in a non-coring arrangement, and also teach novel drilling methods that greatly accelerate the speed of drilling action and substantially reduce bit breakage and change-over downtime. However, in earth structures that include shale, mud seams and other broken and soft formations, the HCD non-coring drill bit of my prior invention easily drills through but tends to plug and the cutting inserts may even shatter in working through stratas of extremely hard, broken and muddy earth conditions.
In a typical roof bolting operation, a series of 4 foot to 6 foot holes having a diameter of 3/4 inch to 2 inches (or more) are drilled in the tunnel roof to receive bolts for anchoring roof support structures. In the past using tungsten carbide bits, frequently only a single 4 foot hole might be drilled before the bit became dull or broken. My prior invention of non-coring PCD insert drill bits (as disclosed in my prior '022 and '787 patents) was capable of drilling over 100-300 holes of 4 foot depth with a single bit and in shorter times with less thrust than the standard carbide bits in hard rock formations of 22,000-28,000 psi. However, as noted, it has been discovered that this prior non-coring drill bit tends to plug in drilling through mud seams and other soft or broken earth formations. It should be noted also that where long flexible cable roof bolts are used as for some soft earth formations, 12 foot to 24 foot holes are required and it may take up to 30 minutes to drill a single hole using prior art drill bits.